A hydraulic system used in production facilities of a plant, a construction machine, or used for powering various types of floodgate facilities (such as an estuary barrage, a floodgate constructed in a river, or a drainage gate of a dam) is constituted of: a hydraulic circuit and an oil tank. The hydraulic circuit includes: an actuator (a hydraulic cylinder, hydraulic motor, or the like) which actuates equipment in the above-described production facilities or the construction machine, or equipment in the facilities such as the floodgate, barrage and the like; a control valve which controls supply/discharge of hydraulic pressure oil to/from the actuator; and a hydraulic pump which produces the hydraulic pressure oil. The oil tank stores: oil to be sucked by the hydraulic pump which supplies hydraulic pressure oil to the hydraulic circuit; and return oil, that is oil which has been returned from the hydraulic circuit.
In the hydraulic circuit, flushing, in which hydraulic oil from the hydraulic pump is circulated so that the hydraulic circuit is filled with the hydraulic pressure oil, is conducted after the construction of the hydraulic system is completed (hereinafter, this flushing is referred to as “initial flushing”). By this initial flushing, return oil which contains, in the form of air bubbles, air having existed in the hydraulic circuit is returned to the oil tank. If the return oil containing the air bubbles is returned to the oil tank and mixed into stock oil, that is oil which has been held in the oil tank, and the thus mixed stock oil is sucked by the hydraulic pump and then is supplied to the hydraulic equipment as hydraulic pressure oil, the air bubbles in the stock oil may cause cavitation in the hydraulic motor, or may be pressurized in the hydraulic equipment to cause a diesel explosion, resulting in a failure of the hydraulic pump or the hydraulic equipment.
Therefore, once the return oil returned at the time of the initial flushing is mixed with the stock oil held in the tank body, the thus mixed oil has to be kept standing until the air bubbles disappear from the stock oil held in the tank body. However, since the air bubbles contained in the return oil is very small and the oil has high viscosity, it takes a long time for the air bubbles to come up to a surface of the oil to be dispersed into air, so there is a problem that the hydraulic system cannot be operated during this time.
One possible way to be taken to avoid this problem is as follows: using an oil tank exclusively used for flushing, hydraulic oil is circulated until air is removed from the hydraulic circuit and the hydraulic oil containing air bubbles is discarded, and then this oil tank is exchanged to another tank which stores new hydraulic oil. However, in the case where the hydraulic circuit is large and long, such as that in a hydraulic system for operating a floodgate (its piping corresponds to the width of a river), there is a problem that the cost for the hydraulic oil is expensive.
Further, even if air bubbles in the oil tank are completely removed through the initial flushing, as the equipment operates, air enters and contamination occurs. The contamination is, for example, contamination resulting from a work environment or entry of external air, and this contamination may cause a failure of working equipment. Therefore, in the hydraulic circuit, it is necessary to occasionally conduct flushing for cleaning the hydraulic oil in the circuit (hereinafter, this flushing is referred to as “cleaning flushing”). Similarly to the above, if air bubbles contained in return oil at the time of cleaning flushing is mixed with the stock oil, and the oil containing the air bubbles is sucked by the hydraulic pump and circulated as hydraulic pressure oil, a harmful effect is exerted on the working equipment.
Patent Literature 1, which is a conventional art to solve the above problem, discloses an apparatus including: flow adjustment means for adjusting the flow of return oil containing air bubbles and then discharging the return oil to an oil tank; and a bubble removal plate disposed so as to be slightly inclined above a surface of oil which has been stored, the apparatus being configured so that the return oil entering the oil tank is caused to flow down along an upper surface of the bubble removal plate and thereby the air bubbles contained in the return oil are removed.
Meanwhile, Patent Literature 2 discloses an oil tank configured so that air bubbles contained in oil returned to the oil tank after passing through the hydraulic circuit are removed using a cyclone-type air bubble removal apparatus provided in the oil tank.